Method of making sheet metal parts

ABSTRACT

A METHOD OF MAKING SHEET METAL PARTS BY PURCHASING BLANKS AND FOLDING THEM OVER. THE EDGES OF THE BLANKS ARE INDENTED IN THE REGION OF THE FOLD LINE TO COMPENSATE FOR MATERIAL FLOW WHEN THE PARTS ARE FOLDED. THE PARTS ARE MADE BY TRIMMING, USING PUNCHES WITH ENLARGED HEADS WHICH FORM THE INDENTATIONS. THE SHEET IS INDEXED SO THAT THE REAR OF THE PUNCH OVERLAPS THE CUT MADE BY THE ENLARGED HEAD ON THE PREVIOUS STROKE, TO PREVENT DRAW-DOWN BURR. THIS INVENTION RELATES TO MANUFACTURE OF SHEET METAL PARTS WHICH ARE PUNCHED AS BLANKS FROM A STRIP OF METAL AND FOLDED OVER.

United States Patent [72} Inventor Richard P. Davis Sanbornton, N11. 211Appl. No. 767,524 (22] Filed Oct. 14, 1968 [45] Patented June 28, 1971[54] METHOD OF MAKING SHEET METAL PARTS 1 Claim, 11 Drawing Figs. 52u.s.'c|. ..I 12/329, 72/379, 72/339,113/119, 29/417 [51] Int. Cl BZld31/02;

B210 37/02, 821d 53/00 [50] Field 01 Search 72/327, 379, 330, 336, 329,335, 337, 339; 113/119; 29/415, 417

[56] References Cited UNITED STATES PATENTS 3,443,542 5/1969 Klumpp113/119 3,461,708 8/1969 Pepe 3,117,472 1/1964 Mingus etal.

ABSTRACT: A method of making sheet metal parts by punching blanks andfolding them over. The edges of the blanks are indented in the region ofthe fold line to compensate for material flow when the parts are folded.The parts are made by trimming, using punches with enlarged heads whichform the indentations. The sheet is indexed so that the rear of thepunch overlaps the cut made by the enlarged head on the previous stroke,to prevent draw-down burr.

This invention relates to the manufacture of sheet metal parts which arepunched as blanks from a strip of metal and folded over.

PATENT En JUN28 19m 3 5 m! m3 sum 1 UF 2 INVENTOR PAUL LOEWENSTEINMETHOD OF MAKING SHEET METAL PARTS BACKGROUND OF THE INVENTION When asheet metal blank is formed by bending especially if the blank is foldeda full I80, flow of material due to bending increases the edge to edgedimension in the region of the fold. In parts made to close tolerancerequirements, the increase may exceed the tolerance in edge to edgedimension for the part as av whole. Another problem in the production ofsheet metal parts is burr formation, due to wear on the punches, orpresentation of a previously cut edge at too small an angle with respectto the punch.

The principalobject of this invention is to provide a method of makingsheet metal parts which makes it possible to produce folded over parts.to close tolerances and which eliminates burr formation. Another: objectisto provide a method-which eliminatesthe need for frequent sharpeningof the punches. Other objects, advantages, and novel features will beapparent from the following description.

SUMMARY The method here disclosed can be used for making parts of anysize, but is especially suitable for making parts which are so smallthat individual inspection and deburring are not economically feasible.The blanks are made bytrimming a strip of sheet metal with puncheshaving forward head portions somewhatwiderthan their body portions. Thestrip is indexed so that the rear ends of the punches overlap the cutsmade by the head portions on the previous stroke. The portions of thestrip left after punching are sheared into individual blanks in such away that the indentations in their edges, formed by the head portions ofthe punches, be. in the region in which the blank is to befolded overtoform the futished part.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings illustratingtheinvention:

FIG. [is a plan view of the blank of a typical sheet metal part madeaccordingto the new method, herediscloeed;

FIG. 2 is a side elevation of the part of FIG. I'in finished condition;

FIG. 3 isaplan view of atypical punch usedin makingthe P FIG. 4 is afragmentary plan view illustrating the first step in making parts bymeansof multiple punches from a strip of sheet metal;

FIGS. 5 and dare views similar to FIG. 4 illustratingsuccessive steps informing the parts;

FIGS.- 7 and 8 are fragmentary plan views illustrating .the effects ofwear orv rounding of 'the comers of conventional rectangular punch;

FIG. 9 is a fragmentary plan view illustrating analtemative shape ofpunch to be used in the new methodiandf FIGS. wand 11 are fragmentaryplan views illustratingthe effect of clearance between the punch and thedie, when punching: material .withan already cut edge presentedatadifferent angles.

DESCRIPTION OFTHEPREFERRED EMBODIMENT The part, generally indicated bythe numeral. 20, shown-in FIG. 2 isa tab twhich-issubsequently to beclarnpedover the end or edge of a metal .strip. The blank of FIG. 1 hasrectangular. end portions 204 and-b and a central-portionlticlwhich Itis understood that the, depth of notches ll is somewhat exaggerated inthe drawings for clarity. 0n relatively thin stock the notches may be inthe neighborhood of a few thousandths of an inch deep. It is necessaryonly that the depth of the notches be equal to or greater than theexpansion of the part in the bend region due to sidewardflow of thestock in the bending operation. The amount of flow for a given piece ofstock can be readily determined by bending and measuring a sample piece.

The blanks for the parts are punched from a strip 25 of sheet metal bymeans'of a multiple die employing a row of punches of the configurationof the punch 26 illustrated in FIG. I. This punch has arectangular bodyportion 26a and a head portion'26b wider than the body portion. Thepunches are somewhat longer than the blanks to be formed. The punchesare so placed in the die that the body portions 26a are spaced from eachother by-the width of portions 20a, 20b, of the parts. The punches areniounted'in a die which may be of any well-known construction. ltisunderstood that the die is provided, in the customary manner, with afemale portion having mating cavities of the'same configuration as thepunches, and that any customary meansfor advancing and guiding the strip25 may be used.

The first stroke of the punches 26 on the forward edge 25a of the stripproduces slots 27 having widened forward end portions 27:: correspondingin shape to the head portions 26b of the punches. The strip is thenadvanced by a distance equal to the length of one of the blanks 20. Asshown in FIG. 5, the rear endsofthebody portions 260 of the punches nowoverlap the forward end portions 270 of slots 27.

On the next stroke of'the die. slots 28 with widened forward endportions are arecut out by punches. The length of slots'zi"(including-portions 28a) is equalto the length of a blank. The strip isagain advanced by the same distance, bringing slots 28 to the positionshown in FIG. 6, with the rear ends of the punches-overlapping? slotportions 284 as they previously' did slot portions 270. The process isrepeated as many times as-required to useup the strip 25. The strip issheared, by an operation which maybe P rformed in the same die, alongwith dot lines 2,9; producing blanks of the configurationof blinklo.

The method here described is a trimming operation, that is thefinishedparts are made'frorn the portions of stock left after punching,as compared to a-true punching operation in whichthe pieces struck outby the punches form the blanks for the finishedpartsffhe use of-puncheawith enlarged head portions inan overlapping cutting arrangementeliminates burring problems. The more commonwesson for burr formationcan be explained as follows:

In FIG. 1, an'initially rectangular punch 30'mating with arectangulardie cavity 31 is illiis'frated asused to punch a piece ofstockSZE'Ihe stock is indexed between strokes so that the successivecuts are-'end'to end: ltis well known that square or sharp corners onpunches tend to become rounded by wear. The rounding of the corners onpunch 30 is here illustrated as somewhat exaggerated for clarity. Thecut 33 made by the punch t on the "previous stroke has rounded forwardcomers 334. When the punch makes'the next cut, regions 32a of the stockwill form buns on the finished parts; To prevent the formation of bum,it is necessary -to sharpen the punches frequently.

FIG. 8 illustrates the punch of FIG. 7 used to make overlapping-cutawayadvancing the stock so'that successive cuts overlap .in'this manner, itwould appear that the burr problem iseliminated. That knot thecaset'however, because of the occurrence of another type of defectsometimes called "draw down him, which can be'understood by referring toFIGS. I0 and II.

It isnecessary toprovide a certain amount of clearance between a punchand its mating'die'cavity. In FIGS. l0 and It, a punch 33 is shownhaving a side-wall 33aseparated from the corresponding sidewallof-thedie cavity 34 by a gap 35. It is understoodthatthe clearance isgreatly exaggerated in the drawings'for purposes of illustration. InFIG. 10, a piece of stock 36 is shown having a cut edge 36a disposed atright angles to the walls 33a and 34a of the punch and die cavity. Underthese conditions, the punch will make a clean cut in the stock. In FIG.II, a piece of stock 37 is shown wit a cut edge 37a lying obliquely withrespect to walls 334 and 340 when the angle 38 between the cut edge 37aand the walls 330, 34a becomes too small, the stock between 33a and 34atends to roll down between the punch and the die before breaking.forming a burr. The angle at which burr formation starts to occur varieswith the material. Some materials can be cut satisfactorily when theangle 38 is about On other materials, about 45 may be the minimum anglewhich can be tolerated.

When the corners of a rectangular punch have become rounded from wear,and the punch is used for overlapping cuts as illustrated in FIG. 8, theedges of the previous cut in the regions of its forward comers 33a liealmost tangent to the sidewalls of the punch. Under this condition drawdown-burr formation will occur as explained in the previous paragraph.

As a practical matter, it is desirable to make the corners, such ascomers 26c, of a punch with a radius initially, rather than absolutelysquare, to reduce comer wear. In the applicant's method, this can bereadily done, because the points of contact between the sides of thepunch and the forward edge 27c of the previous cut are spaced from thecorners 27b of the cut. Even if the latter are rounded, the stock, inthe region of contact with the sides of portion 260 of the punch, isstill presented at approximately right angles to the punch, so that drawdown-burr formation does not occur.

FIG. 9 illustrates a variation in the form of punch which can be used inperfonning this method. The punch 40 has body portion 40b and anenlarged forward head portion 40a with bevelled forward surfaces 44leading to a front surface 45. The cut 41 has a correspondingly-shapedforward portion 41a. It is understood that the material left by thepunches is sheared into individual blanks so that portions 410 of thecuts lie in the region of the fold line, as previously described withrespect to part 20. At points 42, the cut is at approximately 45 to thesidewalls of the punch. Surface 44 is narrower than body portion 40b sothat a constant cut angle of about 45 is maintained at points 42regardless of wear on the corners of the punch.

The method here described makes it possible to manufacture small partsto close tolerances, free of burrs. The need for sharpening the punchesis reduced to a minimum, thus eflecting a considerable saving in laborand down time, and also prolonging the life of the punches.

Although the method here described is intended primarily for formingparts from sheet metal, it is understood that the method is applicableto other rigid sheet materials having similar characteristics.

lclsim:

l. A method of making parts from a sheet of rigid material whichcomprises successively punching cuts in said sheet, each cut having awider portion and a narrower portion, with the wider portion of each cutoverlapping the narrower portion of the ext cut in succession, at leasttwo rows of such cuts being formed side by side in spaced relationshipleaving an intennediate strip with regions of reduced width to formblanks, and bending the blanks in their regions of reduced width.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3 ,587 ,283 June28 1971 Patent No Dated Inventor(s) Richard P VlS It is certified thaterror appears in the aboveidentified patent and that said Letters Patentare hereby corrected as shown below:

The sheets of drawings and the drawing on the front page format shouldbe canceled and the attached drawing and drawing figure substitutedtherefor.

Signed and sealed this 13th day of March 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM PO-10 0 l10-69) USCOMM DC 00370 nos 9 US. GOVEPNMENYPHINYING OFFICE 10! O 166- 134

